In recent years, display devices such as a liquid crystal display device or a light-emitting device have been developed actively because of the growth in demand for portable devices. In particular, an integration technique in which a pixel and a driver circuit (hereinafter, referred to as an internal circuit) using a transistor formed from a polycrystalline semiconductor over an insulator, has been developed actively, since the technique greatly contributes to miniaturization and lower power consumption. An internal circuit formed over the insulator is connected to a controller IC (hereinafter, referred to as an external circuit) or the like via FPC or the like and the operation is controlled.
A power supply potential of an internal circuit is usually around 10V, while an IC constituting an external circuit prepares a signal having amplitude of about 3 V, since the IC operates at a lower power supply potential than the internal circuit. There is a shift register in which a level shift portion is arranged in each stage so as to operate the internal circuit with accuracy using this signal having the amplitude of about 3 V (for example, Reference 1: Japanese Patent Laid-Open No. 2000-339985 bulletin (pp. 3-6)).
In a clocked inverter of a conventional shift register as shown in FIGS. 9A and 9B, in a case that a signal having a smaller amplitude than a potential difference between a high potential power source (VDD) and a low potential power source (VSS) is input to a gate of an n-channel transistor whose source is connected to VSS and a gate of a p-channel transistor whose source is connected to VDD, when the n-channel transistor is ON, the p-channel transistor which is needed to become OFF cannot be turned OFF completely because of the threshold vale of the transistor, and a through current flows from VDD to VSS, which leads to false operation.